Passive immunization experiments have established that antibodies (Abs) provide sterilizing immunity against infections with HIV and contribute to augmented responses to infection. Six epitopes have been identified that induce protective Abs, including sites in gp41 and gpl20, including the V3 loop, the CD4 binding domain, a CD4-induced epitope in the bridging sheet, and a carbohydrate epitope near the base of the V4 loop. Induction of Abs to each of these epitopes may ultimately be useful as a defensive shield against HIV infection, however, each of these epitopes presents a challenge to vaccine design. Anti-V3 Abs were initially thought to mediate only type-specific neutralization of T cell line-adapted viruses. However, new data show that, although type-specific anti-V3 Abs are induced early after immunization and infection, broadly neutralizing anti-V3 Abs also exist, and primary isolates can be efficiently neutralized with microgram amounts of several anti-V3 monoclonal antibodies (mAbs). Recent studies also suggested that the conformation of the V3 loop might be constrained because the V3 must interact with chemokine receptors, a theory now confirmed by our recent structural studies showing molecular mimicry between the V3 loop and structures in the physiologic ligands of CCR5 and CXCR4; this provides an explanation for the broad and potent cross-neutralizing activity displayed by several anti-V3 mAbs. These structural studies have been performed with anti-V3 mAbs that neutralize primary isolates from clades A, B and F. No human anti-V3 mAbs exist which consistently neutralize Clade C viruses, the HIV subtype that causes approximately 50% of the pandemic. Therefore, we propose in Aim 1 to produce human mAbs that recognize conformational epitopes of V3 from the cells of Clade C-infected subjects, use these mAbs to ascertain the structure of the Clade C V3 loop, and design V3 mimetic immunogens. These, and additional clade B-based V3 mimetic immunogens, will be used in Aim 2, in rabbits, to induce and focus the immune response on broadly neutralizing Abs to conserved conformational epitopes in the V3 loop. Finally, in Aim 3, we will examine how the intra- and inter-molecular interactions of gpl20 and gp41 affect neutralization sensitivity to anti-V3 mAbs.